Manufacture of N-substituted tetrahydro-1,4-oxazines

A process for the manufacture of an N-substituted morpholine substantially free from impurities of the general formula I ##STR5## where R is an aliphatic radical of 1 to 20 carbon atoms, a cycloaliphatic radical of 6 to 12 carbon atoms, an araliphatic radical of 7 to 15 carbon atoms or an aryl radical of 6 to 10 carbon atoms, and X.sub.1, X.sub.2, X.sub.3 and X.sub.4 are identical or different and each is hydrogen or methyl, by cyclizing a compound of the general formula II ##STR6## where R, X.sub.1, X.sub.2, X.sub.3 and X.sub.4 have the above meaning, wherein the cyclizing is carried out in the presence of a dehydration catalyst and a hydrogenation is carried out in the presence of hydrogen and a hydrogenation catalyst respectively at from 100.degree. to 300.degree. C and at pressures of from atmospheric pressure to 250 atmospheres.